1. Field of the Invention
The features of the present invention are useful in the printing arts and more particularly in xerographic printing, e.g., electrophotographic or electrostatographic printing. The present invention relates to bias charging members or bias transfer members and copying devices utilizing such members, and more particularly to such members having a particular multi-layer conductive surface coating.
2. Discussion of Related Art
In the well-known process of electrophotographic or electrostatographic printing, the charge retentive surface, typically known as a photoreceptor, is electrostatically charged, and then exposed to a light pattern of an original image to selectively discharge the surface in accordance therewith. The resulting pattern of charged and discharged areas on the photoreceptor form an electrostatic charge pattern, known as a latent image, conforming to the original image. The latent image is developed by contacting it with a finely divided electrostatically attractable powder known as toner. Toner is held on the image areas by the electrostatic charge on the photoreceptor surface. Thus, a toner image is produced in conformity with a light image of the original being reproduced or printed. The toner image may then be transferred to a substrate or support member (e.g., paper) directly or through the use of an intermediate transfer member, and the image affixed thereto to form a permanent record of the image to be reproduced or printed. Subsequent to development, excess toner left on the charge retentive surface is cleaned from the surface. The process is useful for light lens copying from an original or printing electronically generated or stored originals such as with a raster output scanner (ROS), where a charged surface may be imagewise discharged in a variety of ways.
The described electrostatographic copying process is well known and is commonly used for light lens copying of an original document. Analogous processes also exist in other electrostatographic printing applications such as, for example, digital laser printing or ionographic printing and reproduction where charge is deposited on a charge retentive surface in response to electronically generated or stored images.
To charge the surface of a photoreceptor, a contact type charging device has been used. The contact type charging device includes a conductive member which is supplied a voltage from a power source with a D.C. voltage superimposed with a A.C. voltage of no less than twice the level of the D.C. voltage. The charging device contacts the image bearing member (photoreceptor) surface, which is a member to be charged. The outer surface of the image bearing member is charged with the rubbing friction at the contact area. The contact type charging device charges the image bearing member to a predetermined potential. Typically the contact type charger is in the form of a roll charger such as that disclosed in U.S. Pat. No. 4,387,980, the relative portions thereof incorporated herein by reference.
In contact type charging systems, it is important that the charging member contacts the image bearing member uniformly along the length thereof. Contact charge type rollers therefore typically include a conformable material to maintain the contact with the photoconductive member. In typical printing applications, the A.C. and D.C. voltages are applied to a roll type charger in contact with a photoconductive drum.
U.S. Pat. No. 5,765,077 discloses a charging member for charging a member to be charged includes a base member, a surface elastic member supported by the base member. The elastic member includes a foamed member and a coating layer covering the foamed member. A surface of the charging member has an Asker-C hardness of not more than 55 degrees and an international rubber hardness (IRHD) of not more than 80 degrees.
U.S. Pat. No. 5,666,606 discloses an image forming apparatus including a image bearing member having a photosensitive layer, a surface protection layer having fluorine resin material, and a charging member contactable to the image bearing member to electrically charge the image bearing member. The charging member is capable of being supplied with an oscillation voltage. A peak-to-peak voltage of the oscillating voltage applied across a gap between a surface of the charging member and the surface of the image bearing member is not less than twice a charge starting voltage of the image bearing member in the gap and not more than 1600 volt.
U.S. Pat. No. 5,625,858 discloses a contact charging member to be abutted against a charge-receiving member and supplied with a voltage for charging the charge-receiving member is provided. The charging member includes an electroconductive substrate, an elastic layer and a surface layer disposed in lamination. The surface layer comprises crosslinked polymer crosslinked by irradiation with an electron beam. The surface layer may preferably be in the form of a seamless tube formed of the crosslinked polymer. The surface layer crosslinked by electron beam irradiation is less liable to suffer from transfer of a crosslinking agent or a decomposition product thereof to the charge-receiving member. Accordingly, the charging member shows improved durability and stably uniform charging ability suitable for electrophotographic image formation under various environmental conditions.
U.S. Pat. No. 5,576,805 discloses a charging member contactable to a member to be charged to electrically charge it, the improvement residing in that a micro hardness of an end region, with respect to a longitudinal direction, of the charging member is larger than that in a central region of the charging member.
U.S. Pat. No. 5,529,842 describes a charge roll for electrophotography wherein the surface of the roll is formed from a resin layer containing BaSO4 particles coated with SnO2xe2x88x92x (wherein 0 less than X less than 1) is disclosed. The charge roll may comprise a roll shaft, an electroconductive elastic layer and the surface resin layer. The charge roll may further comprise an intermediate layer formed between the electroconductive elastic layer and the surface resin layer.
U.S. Pat. No. 5,506,745 discloses a device for charging a member. The device includes a roller contactable with the member to charge the member. The roller includes an elongated cylinder defining a central cavity in the elongated cylinder. The cylinder is flexible in a radial direction toward the central cavity. The device also includes an electrical biaser for electrically biasing the roller.
U.S. Pat. No. 5,241,343 discloses a conductive foam rubber roller is used as charging roller, developing roller, toner-removing roller, or transfer roller in an image formation apparatus such as an electrophotographic recording apparatus, and comprises a tubular roller element made of a conductive foam rubber material and having a central bore defined by a solid skin layer having an electric resistivity considerably higher than that of a foam structure of the rubber element, and a conductive shaft on which the roller element is mounted and fixed. End sections of the skin layer are removed from the roller element such that the foam structure thereof is in direct contact with the shaft at end sections of the bore thereof. Alternatively, a conductive disc-like member having a central opening formed therein is inserted onto the shaft to be abutted against an end face of the roller element, whereby sufficient electric contact can be established between the roller element and the shaft.
The operation of transferring developing material from the photoreceptive member to the image support substrate is realized at a transfer station. In a conventional transfer station, transfer is achieved by applying electrostatic force fields in a transfer nip sufficient to overcome forces holding the toner particles to an original support surface on the photoreceptive member. These electrostatic force fields operate to attract and transfer the toner particles over onto the copy sheet or other support surface.
Biased roll transfer systems have been used successfully to accomplish toner transfer. This type of transfer was first disclosed in U.S. Pat. No. 2,807,233 which disclosed the use of a metal roll coated with a resilient coating having an approximate resistivity of at least 106 ohm-cm, providing a means for controlling the magnetic and non-magnetic forces acting on the toner during transfer. Bias roll transfer has become the transfer method of choice in many state-of-the-art xerographic copying systems and apparatuses. Notable examples of biased roll transfer systems are described in U.S. Pat. Nos. 3,702,482 and 3,781,105. Other general examples of biased roll transfer systems can be found in U.S. Pat. Nos. 3,043,684, 3,267,840, 3,328,193, 3,598,580, 3,625,146, 3,630,591, 3,684,364, 3,691,993, 3,832,055 and 3,847,478.
The process of transferring development materials in an electrostatographic system involves the physical detachment and transfer-over of charged particulate toner materials from one surface into attachment with a second surface via electrostatic force fields. The critical aspect of the transfer process focuses on maintaining the same pattern and intensity of electrostatic fields as on the original latent electrostatic image being reproduced to induce transfer without scattering or smearing of the developer material. This difficult requirement is met by careful control of the electrostatic fields that, by necessity, must be high enough to effect toner transfer while being low enough so as not to cause arcing or excessive ionization at undesired locations. Such electrical disturbances can create copy or print defects by inhibiting toner transfer or by inducing uncontrolled transfer of the development materials.
U.S. Pat. No. 4,062,812 discloses a method for extending the electrical life of copolymers used in bias transfer rolls. The patent recognizes that control of, and minimization of, the variations in the resistivity under applied voltages with respect to time is important. Thus, certain salts having a particular geometric make-up which are useful for extending the functional electrical life and electrical stability of materials are incorporated into the materials used in xerographic devices.
U.S. Pat. No. 4,116,894 also discloses compositions and a method for enhancing the electrical life of copolymers used in xerographic devices. The patent discloses a specific method for enhancing the electrical life of butadiene copolymers having solubilized conductivity control agents incorporated therein by varying specified quantities of terminally unsaturated hydrocarbonated nitrites in the butadiene.
There is still a need for improved materials for use as either or both a bias charging member or a bias transfer member surface layer, particularly as to the properties of both electrical performance and abrasion resistance.
It is therefore one object of the present invention to develop a coating for a bias charging member or bias transfer member that has superior electrical performance as well as abrasion resistance.
This and other objects are achieved by the present invention wherein a bias transfer member or bias charging member for a xerographic device includes a support substrate, and a multi-layer conductive surface coating over the support substrate. The multi-layer coating includes a base layer of metal-containing particles in a polymer binder, an intermediate layer of conductive particles in a polymer binder, and a top layer of conductive particles in a polymer binder, and the amount of the conductive particles in the top layer is greater than the amount of the conductive particles in the intermediate layer. This multi-layer coating exhibits ideal electrical and abrasion resistance performance for use in bias charging or transfer members.